Udi Dahan has posted an interesting article for anyone thinking about writing unit tests when implementing Test Driven Development (TDD) here. In the article he explains how the use of mock objects allows test writers to de-couple the implementation of the test code from dependent objects and thus enabling you to have a true "Unit Test".
For more information on mock objects and Udi's use of :
Mock mock = new DynamicMock(typeof(Something)); // create a mock
check out NMock, a mock object framework for .NET.